I did some testing with these at Daytona throughout the week and here's what I got:
GTO - 579hp 1182kg 2.0pwr 193mph 47sec laps
Superbird - 616hp 1323kg 2.11pwr 194mph 47s
Mach 1 - 515hp 1309kg 2.5pwr 188mph 48s
GT350R - 561hp 1055kg 1.85pwr 197mph 46s
Cuda - 553hp 1282kg 2.28pwr 194mph 47s
Cougar - 547hp 1181kg 2.12pwr 193mph 47s
Chevelle - 631hp 1329kg 2.07pwr 183mph 49s
Super Bee - 637hp 1389kg 2.14pwr 195mph 47s
Charger - 593hp 1343kg 2.22pwr 187mph 49s
Challenger - 553hp 1398kg 2.48pwr 190mph 48s
Camaro SS - 555hp 1166kg 2.06pwr 185mph 49s
Trans Cammer - 580hp 1470kg 2.49pwr 200mph 46s
Camaro Z28 - 503hp 1173kg 2.29pwr 188mph 48s
Another car that I pulled off the list that I am re-thinking is:
Buick Special - Under 600hp 1100something kg's 198mph 46s
The Cars from GM are unfortunatly weak, so I think the Buick Special may be in order here.
Another thing to note, I used full mod cars with Stages 3's. So I had turn remove the Turbo Kits and Racing Exhausts. And GTO_malloc was riding in a few pure stock cars and adding tuner parts. He was at full weight with increased Horse Power. And running about a 1/2 second faster with this method of tuning.
Edit: So I went ahead and added the Special to GM. I wasn't making people necessarily stick with a car throughout the whole series, but I would like to see each guy running maybe just one make/manufacturer. That would break down kinda like this:
Ford
GT350R, Trans Cammer or Cougar for Daytona
Mach 1 for Road Courses
GM
Special or GTO for Daytona
Camaro SS, Camaro Z28 or Chevelle SS for Road Courses
Chrysler
Superbird, Cuda or Super Bee for Daytona
Challenger or Charger for Road Courses
And for Indy Oval, typically aerodynamic cars work best here so the road course cars are easier to handle at Indy. But if you can tune any of the faster Daytona cars very loose to run just about full throttle for Indy without getting loose into the wall you will be good. Refer to a list of Indy cars I have listed earlier in this thread.